1. Field of the Invention
The present invention relates to impulsive sources, and specifically to high efficiency long lifetime sparker sources.
2. Background Information
Impulsive sources in liquids are important in a wide variety of military, industrial, academic, medical and environmental applications. Impulsive sources produce strong pulsed pressure oscillations and, in some cases, pulses of light, ions, electrons and chemical species. Impulsive sources in air and other media, although not generally in use, may have applications where the impulsive output is useful.
A variety of impulsive sources are known in the art. Explosives are strong and efficient impulsive sources but are limited to a single pulse per source. Due to safety concerns and environmental laws, explosives are not widely used outside the military. Air guns use compressed air to generate impulses, but are relatively inefficient, sensitive to water depth, and have not seen widespread use.
Sparker impulsive sources employ pulses of electrical energy deposited into a liquid (or other medium) to generate an impulse. Sparkers have one or more electrodes, which are important in determining the performance of sparker systems. Furthermore, sparker impulsive sources can be repetitively pulsed and have found commercial application in biofouling control, oil exploration and lithotripsy. Military applications include active sonar, environmental measurements, and mine and submarine countermeasures.
One representation known in the art (U.S. Pat. No. 6,018,502) employs a coaxial sparker in which the center electrode is a solid, similar to the end of a coaxial cable (i.e. a xe2x80x9csinglexe2x80x9d annulus configuration). However, the xe2x80x9csinglexe2x80x9d annulus limits the useful surface area of the inner electrode, limits lifetime and limits practical power.
Sparkers also generate a plasma and/or hot vapor that emits light. When operated in water, sparkers also produce OH radicals, electrons, ions and ultraviolet light that, when combined with the pressures generated, are useful for processes such as decontamination, disinfection, treating organically contaminated water and cleaning surfaces.
In addition, various electrode systems of sparkers known in the art have different limitations. One configuration employs a single metal electrode with the ocean acting as the second electrode, leading to large energy losses and inefficient operation. In another configuration, a primary electrode is surrounded by a cage, that acts as the current return, which also is inefficient in generating impulses. In another, a pair of opposing metal electrodes erodes over time. Since the efficiency of sparkers is sensitive to the electrode gap, performance is degraded by erosion.
In general it is desirable to have an electrode system that allows for rapid turn-on, is robust mechanically, minimizes electrical energy losses and has a high efficiency. Thus in order to be able to operate a sparker efficiently over a long period of time, it would be advantageous to maintain a constant gap between electrodes. Alternatively, it would be advantageous to operate a sparker in such a way that its efficiency is insensitive to electrode erosion.
Also, the impulse from each sparker is omnidirectional, so that in applications with an intended target region, acoustic energy is wasted. A means to recapture or redirect wasted energy is desirable. An acoustic reflector and/or enclosure can improve the utilization of sparker energy.
Accordingly, the present invention provides efficient operation of sparker impulsive sources with sparker heads that maintain a constant gap between electrodes or are insensitive to the electrode gap, and the present invention provides reflectors or enclosures for efficient utilization of impulsive output from the sparker.
The foregoing and other objects and advantages of the present invention are achieved by providing sparker heads with configurations that maintain a constant electrode gap or employ means for high efficiency operation that are insensitive to the electrode gap and/or employ acoustic reflectors and/or enclosures that direct the sparker output to meet requirements for specific applications.
In a sparker a pulsed electrical discharge produces a pressure pulse. In many sparkers known in the art, electrical energy is stored in a high voltage capacitor. A switch between the capacitor and sparker is then closed, applying high voltage to the electrode(s). In order to produce a strong impulse the electrical discharge must first initiate an electrical xe2x80x9cbreakdownxe2x80x9d. Sparkers that use a single electrode, and that utilize the ocean as the second electrode, are very inefficient because of losses to the xe2x80x9cocean electrode.xe2x80x9d Even in sparkers with two or more electrodes the initiation process can consume a large fraction of the energy stored in the capacitor and slow down the discharge, both of which decrease the efficiency of generating the impulse. In sparkers with two electrodes there is an optimum electrode spacing that depends on the capacitance, the charging voltage and the configuration of the sparker head.
In some instances the optimum electrode gap is small, ranging from less than {fraction (1/64)} to xc2xd inch. Furthermore, the optimum performance is sensitive to the gap. In some instances changing the electrode gap by as little as {fraction (1/128)} inch can significantly decrease efficiency. In applications where the sparker operates for many pulses, electrode erosion is a problem.
Consequently, sparker heads that maintained a constant electrode gap would be advantageous for maintaining performance. Alternatively, methods that increased the optimum gap separation, making performance insensitive to gap separation, also would be advantageous for maintaining performance.
One aspect of the invention is to employ a number of inventive arrangements that maintain a constant gap between electrodes. These arrangements have in common the use of parallel metallic electrodes that are electrically isolated except for exposed ends where the electric discharge takes place. In some embodiments a solid non-electrically conducting material is interposed between the electrodes whereas in others the electrodes have a non-electrically conducting coating and are supported and held in position by a base that maintains the electrode gap.
Alternatively, a second aspect of the invention is the injection of an external material between the electrodes. This increases the optimum gap up to several inches, with performance relatively independent of the electrode gap. Furthermore, in many instances electrode erosion is decreased. Consequently, efficient sparker performance is maintained for long operating periods without the need to replace the electrodes. The injected materials may be conductive, in the form of a wire, for instance, or may be a gas or gas mixture. In some instances, the material type and dimensions of wire may be chosen to produce an exothermic reaction and thus increase the acoustic performance. Furthermore, a mixture or slurry of exothermic material with gas or liquid may be used to increase the impulse.
A third aspect of the invention is to employ an acoustic reflector or enclosure to redirect acoustic and energy in a useful manner. The reflector may be a separate arrangement or be an integral part of an enclosure shroud or processing chamber. The reflector may be associated with individual sparkers, or with an entire array.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to illustrative embodiments, the drawings, and methods of use, the present invention is not intended to be limited to these embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be defined as only set forth in the accompanying claims.